Production of paper and paperboard

Abstract

Paper or paper board is made by passing an aqueous cellulosic suspension through a centriscreen or other shear device and then draining the purified suspension, and an improved combination of retention, drainage, drying and formation is achieved by adding to the suspension an excess of high molecular weight linear synthetic cationic polymer before shearing the suspension and adding bentonite after shearing.

We claim: 
     
       1. A process in which paper or paper board is made by forming an aqueous cellulosic suspension, passing the suspension through one or more shear stages, said shear stages selected from the group consisting of cleaning, mixing and pumping stages, draining the suspension to form a sheet and drying the sheet and in which the suspension that is drained includes organic polymeric material and inorganic material, characterised in that the inorganic material comprises bentonite which is added to the suspension after one of the said shear stages in an amount of at least about 0.03%, and the organic polymeric material comprises a substantially linear synthetic cationic polymer flocculant or retention aid having molecular weight above 500,000 and a charge density above about 0.2 equivalents of cationic nitrogen per kilogram of polymer which is added to the suspension before that shear stage in an amount of above about 0.03%, based on the dry weight of the suspension. 
     
     
       2. A process according to claim 1 in which said cleaning stage is a centrisceen, said pumping stage is a fan pump and said mixing stage is a mixing pump. 
     
     
       3. A process according to claim 1 in which the one or more shear stages comprises a centriscreen, the synthetic polymer is added to the suspension before the centriscreen and the bentonite is added after the centriscreen. 
     
     
       4. A process according to claim 1 in which the synthetic polymer is a cationic polymer selected from the group consisting of polyethylene imine, polyamine epichlorhydrin products, polymers of diallyl dimethyl ammonium chloride, and cationic acrylic polymers. 
     
     
       5. A process according to claim 1 in which said suspension contains less than about 0.5% cationic binder and said synthetic polymer is added in an amount of from 0.06 to 0.2%. 
     
     
       6. A process according to claim 1 in which the bentonite is added as a hydrated suspension obtained by dispersing powdered bentonite in water. 
     
     
       7. A process according to claim 1 in which the bentonite is added in an amount of from 0.03 to 0.5%. 
     
     
       8. A process according to claim 1 in which the suspension that is dewatered is substantially free of filler or includes filler substantially all of which was added before the synthetic polymeric material. 
     
     
       9. A process according to claim 1 in which the synthetic polymer is a cationic polymer having intrinsic viscosity above 4 dl/g and formed from acrylic monomers comprising dialkylaminoalkyl(meth)-acrylate or -acrylamide (as acid or quaternary salt). 
     
     
       10. A process according to claim 1 in which the cationic polymer has a cationic charge density of 0.35 to 2.5 equivalents of nitrogen per kilogram polymer. 
     
     
       11. A process according to claim 1 in which a reactive size is incorporated in the aqueous suspension. 
     
     
       12. A process according to claim 1 in which a reactive size is incorporated in the aqueous suspension and in which the synthetic polymer and the reactive size are provided as a dispersion of substantially anhydrous particles of the polymer in a substantially anhydrous oil phase comprising the size and this dispersion is mixed into water. 
     
     
       13. A process according to claim 1 comprising forming an aqueous cellulosic suspension which is substantially unfilled or contain filler, cleaning the suspension by passage through a centriscreen, draining the suspension to form a sheet and drying the sheet, and in which synthetic, substantially linear synthetic cationic polymer is added to the suspension before the centriscreen in an amount of from about 0.03% to 0.2%, based on the dry weight of the suspension, and 0.03 to 0.5% bentonite is added after the centriscreen, and in which said synthetic polymer flocculant or retention aid is selected from the group consisting of polyethylene imine, polyamine epichlorhydrin products, polymers of diallyl dimethyl ammonium chloride, and cationic acrylic polymers. 
     
     
       14. A process according to claim 1 in which the suspension has a solids content of below about 2% at the time the polymer is added to the suspension. 
     
     
       15. A process according to claim 1 in which the said polymer is added before the last point of high shear and the said bentonite is added after the last point of high shear. 
     
     
       16. A process according to claim 4 in which the synthetic polymer is added in an amount of from 0.06 to 0.2%. 
     
     
       17. A process according to claim 16 in which the bentonite is added in an amount of from 0.03 to 0.5%. 
     
     
       18. A process according to claim 9 in which the cationic polymer has a cationic charge density of 0.35 to 2.5 equivalents of nitrogen per kilogram polymer. 
     
     
       19. A process according to claim 18 in which the synthetic polymer is added in an amount of from 0.06 to 0.2%. 
     
     
       20. A process according to claim 10 in which the polymer molecular weight is at least about one million. 
     
     
       21. A process according to claim 1 in which the polymer molecular weight is at least about one million.